1. Field
Embodiments described herein relate in general to tapered slot antennas and, more particularly, to a method and apparatus for improved D-Plane polarization in such antennas.
2. Description of Related Art
During recent decades, antenna technology has experienced an increase in the use of antennas that utilize an array of antenna elements, one example of which is a phased array antenna. Phased array antennas have many applications in commercial and defense markets, such as communications and radar systems. In many of these applications, broadband performance is desirable. Some of these antennas are designed so that they can be switched between two or more discrete frequency bands. Thus, at any given time, the antenna operates in only one of these multiple bands. However, in order to achieve true broadband operation, an antenna needs to be capable of satisfactory operation in a single wide frequency band, without the need to switch between two or more discrete frequency bands. One type of antenna element that has been found to work well in an array antenna is often referred to as a tapered slot antenna element.
Phased Arrays have several primary performance characteristics including bandwidth, scan range, and polarization. Bandwidth is the frequency range over which an antenna provides a good enough match and gain for useful operation. Wider bandwidths generally require some form of balun structure for current balancing at the base, as well as some form of impedance matching structure to permit good energy transfer to and from the feed circuit over the band of operation.
Scan range (or field of view), refers to the range of angles, beginning at boresight or normal to the array plane, over which phasing of the relative element excitations can steer or scan the array beam. Scanning for a linear element polarization is often referred to as being in the electric field plane (E-plane), magnetic field plane (H-plane), or diagonal plane between the electric and magnetic field orientations (D-plane). Maximum scan range is primarily set by the antenna element or “cell” spacing relative to the wavelength at the high end of the band.
Polarization refers to the orientation or alignment of the electric field radiated by the array. An ideal array of elements has a fixed E-field alignment for all, elements, over both the frequency bandwidth and the scan range. This polarization may be linear (a fixed orientation), circular (a specific superposition of polarizations), or many states in between. A dual-polarized array has essentially two co-located antenna elements at each point of the array which can function independently.
Usually the maximum allowable cell spacing is determined by the desired scan angle coverage at the maximum frequency of operation. Once cell size is specified, matching to a desired minimum frequency is achieved by increasing the element length to allow for an impedance taper. However, element length causes vertical currents which influence polarization, regardless of the element type. E-plane and H-plane scans are usually not affected. But in the D-plane scan, the polarization of a tapered notch element does not remain linearly oriented to the notch gap, and is with the change in polarization increasing as scan angle increases. This effect is magnified as the element length increases, or as the separation between the minimum and maximum frequency of operation is increased.
Existing designs and design techniques have not been able to provide a tapered slot antenna element which compensates for D-plane polarization instability without sacrificing gain, bandwidth, scan volume, or manufacturability of the array.